The United Kingdom on Tuesday banned airlines from flying Boeing 737 Max 8 planes into or out of its airports as global pressure mounted to halt flights of the U.S. aircraft giant's hottest-selling model.

There has been pushback in the U.S. as well. Sen. Mitt Romney, R-Utah, called on the FAA to ground the planes "out of an abundance of caution for the flying public" until safety can be assured.?Sens.?Dianne Feinstein, D-Calif., and?Richard Blumenthal, D-Conn., also called for the FAA to ground the MAX 8.

The FAA, however, on Tuesday stood by its earlier determination?of “continued airworthiness” for the Boeing 737 MAX fleet, Marcia Alexander-Adams, an agency spokeswoman, said in an email.

(Pictured) Rescuers work at the scene of an Ethiopian Airlines flight crash, south of Addis Ababa, Ethiopia, on March 11.

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Mourners of victims of the Ethiopian Airlines crash react during the mass funeral in Addis Ababa, Ethiopia, on March 17.

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Relatives mourn next to the coffins of Ethiopian passengers and crew members, during a memorial service for the victims in Addis Ababa, Ethiopia, on March 17.

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A woman kisses a portrait photo as she mourns during the burial ceremony of the Ethiopian Airline Flight ET 302 crash victims in Addis Ababa, Ethiopia, on March 17.

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A woman lies on the coffin of her loved one during a memorial service for the Ethiopian passengers and crew who perished in the crash on March 17, in Addis Ababa, Ethiopia.

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Mourners of victims of the Ethiopian Airlines crash react beside a funeral car during the mass funeral at Holy Trinity Cathedral in Addis Ababa, Ethiopia, on March 17.

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Mourners react during the mass funeral at Holy Trinity Cathedral in Addis Ababa, Ethiopia, on March 17.

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A woman holds a picture of her loved one during a memorial service in Addis Ababa, Ethiopia, on March 17.

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Portraits of victims of the Ethiopian Airlines crash are held beside funeral cars during the mass funeral in Addis Ababa, Ethiopia, on March 17.

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Milka Yimam (C), who lost her 27-year old son, Sidrak Getachew, holds a white rose beside her friend Yamrot Takleson (L) during a special service at Ethiopian Church in Nairobi, Kenya, on March 17.

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Portraits of victims are displayed during the mass funeral at Holy Trinity Cathedral in Addis Ababa, Ethiopia, on March 17.

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An Ethiopian relative of a crash victim mourns and grieves next to a floral tribute at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, south-east of Addis Ababa, in Ethiopia Friday, March 15.

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Recovery work continues at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed, near Bishoftu, south-east of Addis Ababa, in Ethiopia, March 15.

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Chinese relatives of a crash victim mourn at the site of the Ethiopian Airlines Flight ET302 crash on March 15, near Bishoftu, south-east of Addis Ababa, in Ethiopia.

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This photo provided by by the French air accident investigation authority BEA on Thursday, March 14, 2019, shows one of the black box flight recorder from the crashed Ethiopian Airlines jet, in le Bourget, north of Paris. The French air accident investigation agency has released a photo of the data recorder from the crashed Ethiopian Airlines jet. The agency, known by its French acronym BEA, received the flight's data recorder and voice recorder Thursday.

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Forensics experts comb through the dirt for debris at the crash site of the Ethiopian Airlines operated Boeing 737 MAX aircraft, at Hama Quntushele village in Oromia region, on March 14, 2019.

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Ethiopian relatives of crash victims mourn at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, south-east of Addis Ababa, in Ethiopia, Thursday, March 14, 2019.?

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Ethiopian relatives of crash victims mourn and grieve at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, south-east of Addis Ababa, in Ethiopia, Thursday, March 14, 2019.?

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Family members of the flight's main pilot, Captain Yared Getachew, carry photographs of him as they mourn at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, south-east of Addis Ababa, in Ethiopia, Thursday, March 14, 2019.?

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An Ethiopian relative of a crash victim throws dirt in her own face after realizing that there is nothing physical left of her loved one, as she mourns at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, south-east of Addis Ababa, in Ethiopia, Thursday, March 14, 2019.?

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Ethiopian relatives of crash victims mourn and grieve at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, south-east of Addis Ababa, in Ethiopia, Thursday, March 14, 2019.?

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Candles burn before a flower-adorned memorial arch erected at the site of the Ethiopian Airlines Flight ET302 crash on March 14, 2019, in Ejere, Ethiopia.?

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Two men carry suitcases containing the flight recorders from the Ethiopian jet, into the French air accident investigation authority, Thursday, March 14, 2019, in Le Bourget, north of Paris.?

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Forensic experts work at the crash site of the Ethiopian Airlines operated Boeing 737 MAX aircraft, at Hama Quntushele village in the Oromia region, on March 13, 2019.?

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A bouquet of flowers is placed in front of a pile of debris at the scene of the Ethiopian Airlines Flight 302 crash on March 13, 2019, in Ejere, Ethiopia.

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Relatives react at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, south of Addis Ababa, in Ethiopia, Wednesday, March 13, 2019.?

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Wreaths and floral installations stand next to piles of wreckage at the scene where the Ethiopian Airlines Boeing 737 Max 8 crashed shortly after takeoff on Sunday killing all 157 on board, near Bishoftu, or Debre Zeit, south of Addis Ababa, in Ethiopia, Wednesday, March 13, 2019.

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Investigators and recovery workers inspect a second engine after it is recovered from a crater at the scene of the Ethiopian Airlines Flight 302 crash on March 13, 2019, in Ejere, Ethiopia.

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Ethiopian religious leaders pray at the house of crash victim air hostess Sara Gebremichael, 38, in Addis Ababa, Ethiopia, Wednesday, March 13, 2019.

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Kebebew Legesse, the mother of Ethiopian Airlines cabin crew Ayantu Girmay mourns at the scene of the Ethiopian Airlines Flight ET 302 plane crash, near the town Bishoftu, on March 12.

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Ethiopian policemen search through the passengers belongings at the scene of the Ethiopian Airlines Flight ET 302 plane crash on March 12.

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A man holds passengers' passports found at the scene of the Ethiopian Airlines Flight ET 302 plane crash on March 12.

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A page of a flight crew operations manual is seen at the scene of the Ethiopian Airlines Flight ET 302 plane crash, on March 12.

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Colleagues mourn during a memorial ceremony held for the crew who died in the Ethiopian Airlines accident at the Ethiopian Pilot Association Club in Addis Ababa, on March 11.?

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A relative reacts as he leaves the information center following the Ethiopian Airlines Flight ET 302 plane crash, at the Jomo Kenyatta International Airport (JKIA) in Nairobi, Kenya March 10.

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Wreckage is seen at the site of the Ethiopian Airlines Flight ET 302 plane crash, near the town of Bishoftu, southeast of Addis Ababa, on March 11.

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The United Nations flag is pictured at half mast in memory of the victims of Ethiopian Airlines Flight ET 302 plane crash, in Geneva, Switzerland, on March 11.

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Members of the search and rescue mission look on as a digger searches for dead bodies of passengers at the scene of plane crash.

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Delegates at the United Nations Environment Programme (UNEP) world environmental forum observe a minute's silence in memory of the victims of Ethiopian Airlines Flight ET 302 plane crash, at the United Nations complex within Gigiri in Nairobi, Kenya on March 11.

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Relatives and friends of Sara Gebremichael, 38, a crew leader on the Ethiopian Airlines Flight ET 302 plane that crashed, mourn at her house in Addis Ababa on March 11.

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Mourners attend a memorial service held by an association of Ethiopian airline pilots, at which framed photographs of seven crew members were on display, in Addis Ababa, on March 11.

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Villagers stand at the scene of plane crash.

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Engine parts are seen at the scene of plane crash.

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Ethiopian Federal policemen stand at the scene of plane crash.

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A boarding pass is seen at the scene of plane crash.

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People walk past a part of the wreckage at the scene of the Ethiopian Airlines Flight ET 302 plane crash, near the town of Bishoftu, southeast of Addis Ababa, Ethiopia March 10.

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A man carries a piece of debris on his head at the crash site of Ethiopian Airlines Flight ET 302 on March 10.

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Cabin serviettes are seen at the scene of the plane crash on March 10.

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Passenger Ahmed Khalid (L), who missed his Ethiopian Airlines Flight ET 302 while connecting from Dubai, is received by his father Khalid Bzambur (R) at the Jomo Kenyatta International Airport (JKIA) on March 10, in Nairobi, Kenya.

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People use their mobile phones near the flight information board displaying the details of Ethiopian Airlines Flight ET 302, at the Jomo Kenyatta International Airport (JKIA) on March 10, in Nairobi, Kenya.

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Relatives of the victims involved in a plane crash board a bus at Jomo Kenyatta International Airport, on March 10, in Nairobi, Kenya.

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A Red Cross team works amid debris at the crash site.

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Family members of the victims react to news of the plane crash on March 10.?

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Boeing, in a statement Tuesday, restated its “full confidence in the safety of the MAX” and noted that the FAA has not mandated grounding the planes.?

"Based on the information currently available, we do not have any basis to issue new guidance to operators,” Boeing said.

"External reports are drawing similarities between this accident and the Lion Air Flight 610 accident," the FAA said. "However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions."

The agency said, however, that?it expects to require Boeing to complete Max 8 flight control system enhancements – prompted by the Lion Air crash – by month's end.

The FAA said it was providing technical support to the?Ethiopian Accident Investigation Bureau.?Boeing said it was also aiding the investigation.

The plane was delivered to the airline in November,?had flown only 1,200 hours and had undergone a "rigorous" maintenance check Feb. 4. The?pilot, who had?more than 8,000 hours of flight experience, had issued a distress call and was attempting to return to the airport.

The?"black box" voice and data recorders had been found, raising hopes that investigators?soon learn more details of the crash.?Airline CEO Tewolde GebreMariam told CNN the?pilots told air traffic control they were having “flight control problems” before the crash.?

The stakes for Boeing are high: Airlines have ordered 4,661 more of the planes — the newest version of the 737 and best-selling airliner ever.?

Southwest and?American fly the plane?and both expressed confidence in their fleets.?Southwest, which has 34 of the planes and is adding more, said on Twitter that?the airline had flown 31,000 flights on 737 MAX planes?and plans on "operating those aircraft?going forward."

President Donald Trump weighed in on Twitter but took no position on grounding the planes. Trump said planes have become so complex that "computer scientists from MIT" are required to fly them.

"Split second decisions are needed, and the complexity creates danger," Trump added. "I don’t want Albert Einstein to be my pilot. I want great flying professionals that are allowed to easily and quickly take control of a plane!"